1. Field of the Invention
The present invention relates to a piezoelectric thin-film filter, more specifically, a piezoelectric thin-film filter where a vibration portion in which a piezoelectric thin film is interposed between a pair of electrodes is supported in such a manner that the vibration portion is acoustically isolated from a substrate.
2. Description of the Related Art
There have been proposed various configurations to improve characteristics of a piezoelectric thin-film filter where a vibration portion in which a piezoelectric thin film is interposed between a pair of electrodes is supported in such a manner that the vibration portion is acoustically isolated from a substrate.
For example, Japanese Unexamined Patent Application Publication No. 2000-332568 proposes that in order to restrain irregular vibration generated in a lateral resonance mode, an electrode be formed as shown in a plan view of FIG. 27 so that the external shape of a vibration portion 100 when seen from a thickness direction is a non-square, irregular shape.
International Disclosure 2006/129532 proposes that in order to reduce spurious responses, a vibration portion 110 seen from a thickness direction take the shape of an approximate polygon, and perimeters 114a, 116a, 116s, and 116t of electrodes 114 and 116 be formed into waves so that each side of the approximate polygon takes the shape of a wave that is repeatedly bent or folded, like a triangle wave, a square wave, or a sine wave, as shown in a plan view of FIG. 28.
Also, as shown in a plan view of FIG. 29, US Patent Application Publication 2003/0227366 proposes that in order to reduce ripples in a filter, different aspect ratios should be set for all resonators forming the filter.
If a vibration portion having an asymmetrical shape, as described in Japanese Unexamined Patent Application Publication No. 2000-332568, is formed and Lamb waves are dispersed in the vibration portion, a loss unavoidably occurs and the characteristics of a resonator formed by this method has problems for a filter application. Because, among filter characteristics, the insertion loss is increased and the steepness of the shoulder characteristic of the filter is deteriorated.
Also, if a vibration portion having a perimeter taking the shape of periodic waves, as described in International Disclosure 2006/129532, is formed and Lamb waves are dispersed in the vibration portion, a loss occurs unavoidably and the characteristics of a resonator formed by this method has problems for a filter application. Because, among the filter characteristics, the insertion loss is increased and the steepness of the shoulder characteristic of the filter is deteriorated.
Also, if different aspect ratios are set for resonators as described in US Patent Application Publication 2003/0227366, spurious responses in resonators having a rectangular plan view shape are extremely large. The setting of different aspect ratios is not sufficient to reduce the ripples.